This invention relates in general to gas discharge fluorescent devices, and, in particular, to an improved cold cathode gas discharge fluorescent device. Many of the features of this invention are useful, in particular, for delivering higher intensity illumination.
Hot cathode fluorescent lamps (HCFLs) have been used for illumination. While HCFLs are able to deliver high power, the useful life of HCFLs is typically in the range of several thousand hours. For many applications, it may be costly or inconvenient to replace HCFLs when they become defective after use. It is therefore desirable to provide illumination instruments with a longer useful life. The cold cathode fluorescent lamp (CCFL) is such a device with a useful life in the range of about 20,000 to 50,000 hours.
HCFL and CCFL employ entirely different mechanisms to generate electrons. The HCFL operates in the arc discharge region whereas the CCFL functions in the normal glow region. This is illustrated on page 339 from the book Flat Panel Displays and CRTS, edited by Lawrence E. Tannas, Jr., Von Nostrand Reinhold, New York, 1985, which is incorporated herein by reference. The HCFL functions in the arc discharge region. As shown in FIG. 10-5 on page 339 of this book, for the HCFL functioning in the arc discharge region, the current flow is of the order of 0.1 to 1 ampere. The CCFL functions in the normal glow region. Functioning in the normal glow region of the gas discharge, the current flow in the CCFL is of the order of 10xe2x88x923 ampere, according to FIG. 10-5 on page 339 of the above-referenced book. Thus, the current flow in the HCFL is about two orders of magnitude or more than that in the CCFL.
The HCFL typically employs a tungsten coil coated with an electron emission layer. For more details, see page 61 of Applied Illumination Engineering, Second Edition, Jack L. Lindsey, 1997, published by The Fairmont Press, Inc. in Lilburn, GA 30247, which is incorporated herein by reference. More than 1 watt of power is needed to heat the tungsten coil to about 900xc2x0 C. At this temperature, the electrons can easily leave the electron emission layer and a small voltage of the order of about 10 volts will pull large currents into the discharge. The large current flow is in the form of a visible arc, so that the HCFL is also known as the arc lamp. The small voltage will also pull ions from the discharge which return to the tungsten coil, thereby ejecting secondary electrons. However, since the cathode-fall voltage (xcx9c10 V) is small, the sputtering effect of such ions would be small. The lifetime of an HCFL is determined primarily by the evaporation of the electron emission layer at the high operating temperature of the HCFL.
The CCFL emit electrons by a mechanism that is entirely different from that of the HCFL. Instead of employing an electron emission layer and heating the cathode to a high temperature to make it easy for electrons to leave the cathode, the CCFL relies on a high cathode-fall voltage (xcx9c150 V) to pull ions from the discharge. These ions eject secondary electrons from the cathode and the cathode-fall then accelerates the secondary electrons back into the discharge producing several electron-ion pairs. Ions from these pairs return to the cathode. Because of the high cathode-fall voltage (xcx9c150 V), the ions are accelerated by the cathode-fall voltage from the discharge to the cathode, thereby causing sputtering. Different from the HCFL, no power is wasted to heat the CCFL to a high temperature before light can be generated by the lamp.
The HCFL operates at a relatively low voltage (xcx9c100 V) whereas the CCFL operates at high voltages (of the order of several hundred volts). The HCFL operates at a temperature of about 40xc2x0 C. and above, with the cathode operating at a relatively high temperature of about 900xc2x0 C, whereas the CCFL operates in a temperature range of about 30-75xc2x0 C., with the cathode operating at a temperature of about 80-150xc2x0 C. For further information concerning the differences between HCFL and a CCFL, please see the paper entitled xe2x80x9cEfficiency Limits for Fluorescent Lamps and Application to LCD Backlighting,xe2x80x9d by R. Y. Pai, Journal of the SID, May 4, 1997, pp. 371-374, which is incorporated herein by reference.
CCFLs typically comprise an elongated tube and a pair of electrodes where the current between the electrodes in the CCFL is not more than about 5 milliamps and the power delivered by the CCFLs less than about 5 watts. In order to increase the power delivered by the CCFL, it is possible to increase either the length of (and consequently, the voltage across the CCFL) or the current in the CCFL. It may be difficult to manufacture CCFLs whose tubes are excessively long. Furthermore, when the tube length of the CCFL is excessive, they must be operated at high voltage so that this increases the cost and reduces the reliability of the CCFL drivers. Another way to increase the power output of the CCFL is to increase the current in the CCFL. However, as noted above, because of the high cathode-fall voltage which may be about 150 V, ions are accelerated from the discharge towards the cathode, thereby causing sputtering. This means that if a large current is flowing in the CCFL, the return of the ions to the cathode may cause excessive sputtering, which drastically reduces the useful life of the CCFL.
The metal from the cathode that is sputtered may also combine with the gas medium in the CCFL, such as mercury, to form a mercury alloy on the wall containing the gaseous medium, thereby reducing the amount of mercury present in the medium to the extent that the CCFL may become defective for the reason that there is not enough mercury left for generating gas discharge in the gaseous medium. Furthermore, the heat generated at the cathode will need to be dissipated. Since the cathode and the gaseous medium are typically enclosed in a sealed envelope, it may be difficult for the heat to be effectively dissipated so that the cathode temperature may reach a 110xc2x0 C. or above. Thus the cathode, the gaseous medium and the envelope are all at elevated temperatures which may reduce the useful life of the CCFL. Moreover, the conventional CCFL design requires connecting wires to pass through the envelope to connect the electrodes to a driver, while maintaining a vacuum seal of the gaseous medium within the envelope. This may be costly, cumbersome to produce and reduces the effective yield in production.
For the reasons explained above, CCFLs have not been used as high power illumination systems for delivering high intensities. As noted above, the power delivered by CCFLs is generally less above 5 watts. Even though the CCFL is more efficient than incandescent lamps, the maximum intensity that can be delivered by conventional CCFLs would be less than that generated by a 25 watt incandescent lamp. For this reason, CCFLs have not been used for illumination purposes and have not been used to replace incandescent lamps. On the other hand, CCFLs are much more energy efficient than incandescent lamps and have a much longer useful life. Therefore, it is desirable to provide an improved cold cathode gas discharge system that can be used at high power to deliver high intensity illumination while retaining its advantages of energy efficiency and longer useful life.
For the purpose of delivering high intensity illumination, CCFL designers need to solve two problems: sputtering of the cathode material caused by the cathode-fall voltage and the dissipation of heat.
To reduce the amount of cathode material that is sputtered during the gas discharge, at least one of the electrodes may be removed from the gas discharge medium; preferably, both electrodes are removed so that there is no electrode present in the gas discharge medium and an AC voltage is applied to the gas medium by means of electrically conductive members outside the medium. This would entirely eliminate the sputtering problem.
When the conventional CCFL is operated at high power, large currents will flow between the pair of electrodes in the CCFL, and as noted above, the return of the ions to the cathode may cause excessive sputtering which drastically reduces the useful life of the CCFL. An alternative solution for solving the sputtering problem is to spread out the large current over more than one pair of cathodes so that the amount of current flow through any one particular cathode would be reduced, thereby also reducing the sputtering experienced by each individual cathode. Preferably, current limiting devices may be used to connect the driver to the multiple cathodes.
A cold cathode gas discharge fluorescent device includes at least one cold cathode fluorescent lamp and a driver supplying power to the at least one lamp to cause it to emit light. The driver is typically housed in a housing and a light transmitting container is used to contain the at least one lamp, where the container is connected to and forms a chamber with the housing to house the at least one lamp. Where the at least one lamp is operated at high power, much heat would be generated during the operation so that an important concern is heat dissipation. Heat dissipation may be enhanced by a number of different features some of which may be used separately or in conjunction with one another.
One feature for enhancing heat dissipation is to provide a hole in the housing as well as the container to allow air circulation between the chamber and the environment to dissipate heat generated by the at least one lamp.
Another possible design is to employ a container for the at least one lamp where the container is open at one end to allow better heat dissipation.
Yet another possible design is to omit the container altogether. The container lends mechanical strength to the fluorescent gas discharge device. Where no container is employed at all for housing the at least one lamp, and where the at least one lamp is in the shape of a spiral, means is provided for attaching at least two adjacent rounds of the at least one lamp to one another to increase mechanical strength of the gas discharge device.
Other desirable features of the invention pertain to designs to increase light intensity delivered by the device. Thus in one design, the portion of the housing proximate to the container is larger in dimensions than the portion of the housing distal from the container. This permits a larger container to be used for housing a longer and/or larger or multiple cold cathode fluorescent lamps.
In another design, the at least one lamp has a cylindrical envelope substantially in the shape of a spiral. The container has a first section proximate to the housing and a distal second section away from the housing larger than the first section. This permits the container to hold a spiral lamp of larger diameter. The device preferably has a light emitting window that is larger than 50% of the area enclosed by the spiral.